Here is a ragtag assortment of odds and ends from Apple III, thrown
together almost untainted by logical sequence. Some have already been
published elsewhere; some are obtainable in the fine print of Apple manuals;
and some are the fruit of personal investigation. Accuracy, particularly
in the latter category, may not be uniformly high. So be warned.

Let's face it. Extracting information from Apple Computer isn't the
easiest thing in the world. In fact, it's usually faster, and more fun, to ask
the Apple III itself. There is no obvious reason for Apple's reticence. The
folks at Apple intend, they say, to publish the SOS Reference Manual
and, eventually, the Driver Writer's Guide. The reference manual exists
already, more or less, as a textbook for the Apple III Technical Workshop.
If you're keen on writing assembly language for the III, by all
means take that course. It tells you lots and lots, although not quite
everything you might wish to know. Inquiries, however, seem to drift off
into never-never land.

Why so secretive? The effect seems primarily to impede the efforts of
would-be Apple III programmers, which you might suppose would not
be in Apple's interest. Maybe they are protecting something else? The
techniques of RAM-based operating systems may have a more general
applicability... perhaps to the Lisa?

No doubt a recognized software development firm, prepared to sign
certain agreements, can obtain source materials and technical assistance.
But many bright ideas must incubate and grow and be played with on
the machine before they become sufficiently clear and explicit to warrant
a formal approach to Apple. A lot of maybe-we-coulds must simply have
vanished because the programmer had insufficient information to permit
experimentation.

A Memory Map. At any one time, the 6502 cpu works with 64K addresses arranged as follows:

0000..1FFFlower s-bank

2000..9FFFuser bank 0-6

A000..FFFFupper s-bank

The system bank is always on-line. It contains SOS.Kernel and other
goodies. The user banks are switched in and out. Only one user bank is
on-line at any given moment.

Table 1 describes the function of pages in the lower system bank.

Table 1. Lower system bank: pages $00..1F.

00:

"True" zero page. Used early in boot sequence, and as the zero page for interrupt handlers.

01:

"Normal" 6502 stack. Addressed by PHA, JSR, and so on, whenever bit 2 of environment register ($FFDF) is set. Used as stack page by interrupt handlers, drivers, and by SOS.Kernel itself.

Upper System Bank: $A000..FFFF. SOS.Kernel occupies
$BC00..FFFF. In the future SOS.Kernel may get longer and extend
down as far as $B800. SOS.Interp, which is "absolute" code, is normally
loaded below SOS.Kernel. Actually, it is loaded into the highest user
bank (bank 6 in a 256K machine) beginning at some predetermined location
($7600 for Pascal). It may then extend upward for any length, up
to the lower end of SOS.Kernel (presently $BC00). Thus it usually overlaps
from bank 6, a user bank, into system bank $A000..BBFF. This is
important because the overlap gives the interpreter a sizable area in system
bank for code that is always on-line. Bank-switching must always be
done from system bank. If you switch banks while running in a user
bank -- puuff! Suddenly you aren't.

A very short interpreter might lie only in user bank or only in system
bank. The loading site and length are determined by the writer when the
interpreter is created. It is absolute code ".org'd" on the intended loading
site.

Usually the upper system bank is all RAM, except for
$FFD0..FFDF and $FFE0..FFEF, which are the onboard D and E
VIAs (versatile interface adapters). In particular, if bit 6 of the
environment register is clear, then $C000..CFFF is RAM. If that bit is set, then
this area is I/O. There are also $20 bytes of RAM "under" the VIAs at
$FFD0..FFEF. Normally they are off-line. These RAM bytes can be
accessed only by "8F" extended addressing. This small area of RAM is
unique in that it is not disturbed by a control-reset reboot, so that's
where they keep the last valid clock value -- less useful, since you obtained
your functioning clock chip.

The RAM of SOS.Kernel area $C000..FFFF can be write-protected
by setting bit 3 of the environment register. Normally this RAM is
protected while the interpreter is running. This is the user environment, and
Apple doesn't trust you. It is unprotected in the driver, SOS.Kemel,
and interrupt handler environments. Write-protection doesn't affect
I/O $C000..CFFF when that is enabled, nor the VIA registers
$FFD0..FFEF.

Highest User Bank: Bank 6 in a 256K Machine. At boot time
SOS.Interp is loaded here, at whatever site the writer has designated, assuming,
as is usually the case, that the interpreter is not confined entirely
to $A000..BBFF in system bank. Next the drivers are loaded below
SOS.Interp, one after another, in whatever order they are encountered in
SOS.Driver, which is just the reverse of the order listed by the System
Configuration Program. For this purpose a modular driver is just one
driver, no matter how many modular units it may contain. The drivers
extend, if necessary, down to the bottom of bank 6. If more space is
needed, they continue from the top of the next lower user bank ($9FFF
with bank 5 switched in). Each driver, however, must be completely contained
within its bank, so if there is insufficient room for the complete
driver in bank 6 it will be placed entirely in bank 5. Any space left over
below the drivers is free and available for requisition by the interpreter.

Interpreter Strategy. Interpreters should never assume they are resident
in the highest bank, even though that is where they are normally
loaded. Interpreters will run perfectly well in other banks. All you need
to do is copy the user bank portion of the interpreter into the corresponding
bytes in another (free and available) bank. Then switch in that bank
(being careful not to self-destruct) and perform a jump to the first byte of
interpreter code. The interpreter still overlaps into system bank
($A000..BBFF) just the way it always did. Bank-switching affects only
the user bank. Interpreters, therefore, should always find out where they
are by checking the bank register ($FFEF), never by making assumptions
or by using location $1901, which does contain the highest bank
number.

This relocation trick can be used for interpreter-switching schemes. A
small switching interpreter is loaded from disk as the original
SOS.Interp. It is placed entirely in the upper part of the highest user bank,
with the drivers immediately below. The switching interpreter, in turn,
loads in another interpreter (perhaps Pascal) but places the user-bank
portion in a lower bank, where it runs very happily. The switching interpreter
remains in the highest bank, taking up very little room, just waiting
for you to call it back by pressing a special key combination (for
which you will need a small modification of the console driver). Then the
switching interpreter can load in, and run, some other interpreter, such as
Basic.

User Bank 0: "8F" Addressing. The "zero-page anomaly" in Apple
III means that every time the 6502 executes a zero-page instruction it
actually operates on the designated zero page, found as the value of the
zero-page register ($FFD0). The same thing happens when the (sixteen-bit)
operand of an instruction has $00 in the high byte, since that also refers
to zero page. Similarly, in extended addressing, if the place you are
headed has $00 in the high byte of its address, then that is also interpreted
as a zero-page location and you are given whatever is the current zero
page. But that may not be what you want.

Extended addressing looks at a 64K stretch of memory comprising
two consecutive user banks. In extended addressing, $0000 is the bottom
of one user bank and $FFFF is the top of the next higher user bank.
Whichever pair of user banks is active depends on the Xbyte of the extended
address. This works fine except for the lowest page of the lower of
the two user banks in the pair. That would have to be addressed as
$00xx/8b. But the high byte of that address is $00, so you are given zero
page instead.

Normally you get around the zero-page anomaly by decrementing
the Xbyte. Then you are looking at a different pair of user banks, with
your target bank as the higher member of the pair. The lowest page of
that bank can then be addressed as $80xx/8b-1. But what about the
lowest page of bank 0? There is no lower user bank to put underneath it
in a pair. Hence 8F addressing.

8F is an Xbyte which, when present, causes the extended addressing
mechanism to look at 64K of memory constructed as follows:

0000..1FFFlower s-bank

2000..9FFFbank 0

A000..FFFFupper s-bank

It is aimost exactly like system addressing (Xbyte $00), with bank 0
switched in. (There is one other interesting feature. It is all RAM,
including the RAM beneath the VIA registers $FFD0..FFEF.)

Thus if you are doing a lot of talking to user bank 0, you should use
8F as the Xbyte and address the bank as $2000..9FFF, corresponding to
$0000..7FFF in the bank. Then you can get to the lowest page without
worrying about the zero-page anomaly.

User Bank 0: Graphics. Which, of course, is why you'll want to be
talking to bank 0. That's where graphics are, when graphics are allocated.
Pascal and Basic each provide for allocations of $00, $40, or $80
pages of graphics, depending on the graphics mode. Pascal also allows
$20 pages, which is enough for one lo-res black-and-white buffer. Hi-res
mode appears to interleave two lo-res modes in alternate columns or
groups of pixels, much as eighty-column text interleaves two forty-column
screens.

The number of pages allocated for graphics is stored in location
$1907 in the SOS data area. Presumably this byte is used by the video
generator apparatus, as are surrounding bytes in that area.

In black-and-white lo-res mode (BW280), buffer 1 runs from
$2000/208F..3FFF/3F8F (which is $0000..1FFF in bank 0). Buffer 2 is
found in $4000/408F..5FFF/5F8F. In buffer 1 the lowest byte ($2000/208F)
represents the upper-left corner of the screen. Each bit represents
one pixel. Successive bytes (and their contained bits) are in order
from the left edge of the screen. One accesses individual bytes by indirect
Y-indexed addressing (extended addressing) off the base address, which
is the leftmost byte in that horizontal row. The following algorithm (for
BW280, buffer 1) relates corresponding bytes in successive rows. It was
discovered empirically and is doubtless pathetically slow:

In hi-res mode (BW560) altemate bytes (groups of eight pixels) come
from corresponding bytes of the two lo-res buffers just discussed. Thus
the sequence is $2000, $4000, $2001, $4001....Hi-res buffer 2 is the
corresponding structure beginning at $6000/608F. In color mode (CP280)
the "upper lo-res buffer" presumably contains color information. We are
not sure about COL140 mode. And we are not sure if the base address
algorithm holds for these modes.

When you are ready for the video generator to display your graphics,
it is necessary to fiddle with the soft switches (see table 8). Graphics
information is always taken from bank 0, regardless of which user bank
is switched in. Presumably this is hard-wired, although it is just
conceivable that the source bank is software-selected.
If so, we don't know how.

The Text Pages: $0400..07FF and $0800..0BFF. Apple III text
memory is very similar to Apple II; possibly identical for forty-column
mode. In eighty-column mode the two "text pages" are interleaved: even
columns from $0400..07FF, odd columns from $0800..0BFF. The reason
for this peculiar arrangement is found in the direct memory access
(DMA) apparatus of the video generator. When Apple III was designed
for an eighty-column display, the video generator had to call up twice the
amount of information as it did for the forty-column display of Apple II.
But it did not have twice the time in which to do it. So the memory-fetch
path in Apple III was made sixteen bits wide. Every data fetch actually
gets two bytes. The video generator uses both. The 6502 chip uses one
and ignores the other (except in the case of the Xbyte, which is that extra
byte used in extended addressing). The memory fetch does not get two
adjacent bytes. It gets the byte at address and the byte at address: high
byte XOR $0C. Thus a fetch to $0400 also gets the byte from $0800,
which the video generator puts in the odd column. And this is also the
reason why the Xbyte page is related to its zero page by the same relation,
high byte XOR $0C.

The Apple II "screen holes" are there, but you aren't supposed to use
them for peripheral card scratchpad space. In the Apple III these locations
are used as transfer ports when downloading character sets to the
video generator. But downloading occurs only at boot time or when programs
deliberately change character sets. It is relatively rare. The rest of
the time these locations seem to be idle. It may be that a peripheral card
could use them for a while. But it's illegal according to the definition of
Apple III.

It is possible to write directly to the screen from assembly, bypassing
the console driver. Just put ASCII codes in the appropriate memory locations.
The high bit should be clear for inverse and set for normal, assuming
you are using a standard (not inverted) character set.

The bytes in each horizontal line are accessed by X-indexed addressing
off the base address, which is the leftmost byte of that line (see table 2).

If column is odd, add $0400 to the address.
Use X index := column DIV 2;

Table 2. Text screen line numbers versus base addresses.

00 0400

08 0428

10 0450

00 0400

08 0428

10 0450

01 0480

09 04A8

11 04D0

02 0500

0A 0528

12 0550

03 0580

0B 05A8

13 05D0

04 0600

0C 0628

14 0650

05 0680

0D 06A8

15 06D0

06 0700

0E 0728

16 0750

07 0780

0F 07A8

17 07D0

In eighty-column mode use X := column DIV 2. If the column number
is odd, you must also add $0400 to the base address given in the table.
Alternatively, the base address and index may be computed with a modification
of the Apple II subroutine Bascalc (table 3).

The Character Set: $0C00..OFFF. At boot time the system character
set is loaded from SOS.Driver and stored in these pages. Similarly, if
you download another character set from a program by issuing a
DControl call #16 or #17 to the console driver, the new set is also placed
here. But these pages are not the active character set in current use by the
video generator. This is merely a staging area. From these four pages the
character set is further transferred to the video generator's storage area,
wherever that is. It is not in addressable memory. Presumably the machine
contains a 1K RAM chip dedicated for this purpose, analogous to
the ROM chip beneath the Apple II keyboard that contains the character
set for that machine. In any event, you can change the copy in
$0C00..0FFF all you wish, but nothing happens.

The console driver uses a complex mechanism to transfer the.character
set into the video generator. It sets up an interrupt-driven background
program (spooler) by allocating system internal resources (SIR)
numbers $05, $06, and $10. The video-generator mechanism then interacts
with the SIR#06 interrupt handler (embedded in the console driver)
to transfer the character definitions at its leisure. The computer's attention
is returned to the user's programs, and the video generator interrupts
when it feels ready for another swallow. There may be simpler ways
if you are willing to let the main program wait. For an entire character
set the download procedure takes about a second to complete.

The actual transfer involves the E-VIA's peripheral control register
($FFEC), interrupt enable register ($FFEE), a couple of sites in $C000
I/O space ($C0DA and $C0DB) that are probably soft switches, and the
notorious screen holes. Apparently the interrupt handler moves the character
descriptions piecemeal from the $0C00 area to the screen holes and
then alerts the video generator to move them on from there into its own
dedicated RAM.

This transfer could probably proceed just as easily from any memory
buffer to the screen holes; the $0C00 staging area is merely a convenience.
But if you are operating from a background program, and if that
program is the interrupt handler itself, then the buffer must be in system
bank. If the buffer were in a user bank it would surely go off-line due to
bank-switching. Extended addressing is not available for interrupt handlers;
it doesn't work on the true zero page. Hence the $0C00 buffer.

Typehead Buffer: $l500..15FF. Page $15 is set aside for use by the
console driver as a typeahead buffer. It is nothing more than a
first-in-first-out queue. Actually two queues. The first queue ($1500..157F)
contains KBD values, which are the ASCII codes generated by the keyboard.
For each KBD there is also a KBDFLG byte, the second keyboard byte,
which flags the various modifier keys. KBDFLG is stored in
the corresponding byte in the second queue ($1580..15FF). The console
driver maintains a count of the current number of characters in the
queue and keeps index pointers to the current front and rear of the
queue.

When a key is pressed, KBD appears at $C000 just as it does in Apple II.
At this time KBDFLG also becomes available at $C008. The keyboard interrupt
is cleared with the keyboard strobe, $C010, just as it is in Apple II.

KBD and KBDFLG are picked up by the keyboard (SIR#02) interrupt handler,
which is embedded in the console driver. If they represent
one of the five console control keys, that function is executed immediately.
Otherwise, if a standard key was pressed, KBD would be used as
an index into the keyboard layout look-up table (page $17). KBDFLG
and the modified value of KBD are then stored in the typeahead buffer.
The console driver will retrieve them when it feels so inclined.

Before exiting, the keyboard interrupt handler also checks to see if
the "any-key" event is armed or if this is the "attention" event character.
If so, the handler queues up the appropriate "event." Later, before retuming
to the user program, SOS checks the event queue and transfers
control to the event handler as a subroutine.

SOS Data and Jump Tables: $1900..19FF. The first few bytes on
page $19 contain important status information (table 4). During ordinary
business, some (or all) of these bytes control the video generator
and/or similar accessory apparatus. But when the monitor is running,
they have no perceptible effect. So there must be more than one way to
control the video generator.

Page $19 also contains a jump table beginning at $1910. The jumps
take the form "1913: 4C CA E2 JMP E2CA". The table provides fixed
entry addresses for certain subroutines that apparently will be supported
in future versions of SOS. The list is in table 5. Those marked with an asterisk
are documented by Apple and are legal to use. The others... well,
they do appear in the jump table.

Page $19 also contains a copyright notice at $1990, a few other data
bytes of mysterious function, and a lot of zeros. The subroutine SysErr
stores the error number at $1980 and the return address at $19FD and
$19FE. The "system-failure" routine uses the end of this page to store the
program counter and all register values for use in debugging. Other SOS
routines store various temporaries on this page.

The copyright notice at $1990 is a good spot if you want to store
things that can be found from the Monitor. If you want to store a lot of
stuff you can also use the character set area.

Those Registers: the Onboard 6522 VIAs. The two VIAs are referred
to as D-VIA ($FFD0..FFDF) and E-VIA ($FFE0..FFEF) respectively.
They are fully occupied with Apple III hardware manipulations.
You cannot, for example, use the VIA timers for your own purposes.
The VIAs manage bank-switching, zero-page selection, and much
of the other machinery that permits Apple III to accommodate the 64K
address space of the 6502 cpu chip.

$FFEF: Bank register (E-VIA IORA). The low nibble selects the currently
switched-in bank. The high nibble is generally $F. Attempts to
change the high nibble have no effect. Those four bits are flags for interrupt
requests from the slots.

$FFDF: Environment register (D-VIA IORA). Table 6 lists the significance
of its bits. Apple would be happier if you confined your attention to bit 7
and didn't mess with the others. Table 7 contains a variety of
information about the various standard environments.

$FFD0: Zero-page register. Selects the current zero page, which can
be assigned to any page in memory. If alternate stack is enabled (bit 2 of
$FFDF is clear) then all stack-using opcodes use the current zero page
XOR $01. Extended addressing, on the other hand, functions only for
zero pages in the range $18..1F. The user zero page is $1A. That's you
and/or the interpreter. Drivers and SOS.Kernel use $18. Interrupt handlers
use $00. SOS is supposed to decide these things; you are not. The
SOS call handling routine even checks to see if the caller's zero page is
$1A. If not, it crashes the system. Somewhere in darkest Cupertino, Apple
maintains a coven of witch doctors who will cheerfully do unspeakable
things to your image, should you violate this trust.

$FFDD: "Any-slot" interrupt flag. When a peripheral card in one of
the slots pulls down the interrupt line, the interrupt handler is entered
with 6502 interrupts disabled. The interrupt handler is, of course, responsible
for clearing the interrupt condition on the card. If the interrupt
handler wishes to enable 6502 interrupts (as it should if it will run
longer than 500 microseconds) then it must also clear the "any-slot" interrupt
flag by storing $02 in $FFDD. Otherwise the interrupt manager
will do it for you when the handler exits.

I/O Space: $C000..CFFF. I/O space is on-line when bit 6 of the environment
register ($FFDF) is set. It is actually $C000..C4FF and
$C800..CFFF. The intervening bytes $C500..C7FF are always RAM.
Table 8 lists those registers of which we have some clue, There are many
mysterious others. When in doubt, there is a good chance that a register's
function is similar or identical to its role in Apple II.

Notice that the $10 bytes beginning $C080, $C0D0, $C0E0, and
$C0F0 are not used for slots in the III. In Apple II they would be slots 0,
5, 6, and 7 respectively. There may or may not be a clue to their function
in the assignment of various connecting plugs to imaginary slots in emulation
mode. For example, $C0F0+ is the ACIA (asynchronous communication interface
adapter), as indicated in table 9. The ACIA runs the
serial port, which in emulation mode is assigned to an ethereal slot 7.
Similarly, emulation mode assigns the floppies to slot 6, and the floppy
drivers (buried in SOS.Kernel at $E899) probably access bytes in
$C0E0..C0EF, and in $C0D0..C0DF as well. But this may only be speculation.

$C800..CFFF is a 2K peripheral card expansion space used "in common"
by all the slots. As in Apple II it can be selected by referencing one
of the peripheral card I/O locations assigned to that slot. $CFFF does
deselect all slots, but $C020 (formerly the cassette output toggle) is the
preferred Apple III deselection strobe.

There are some new rules for using $C800 space that are intended to
mesh with Apple III's interrupt-driven operating system. You are supposed
to allocate the space prior to use by calling the SOS subroutine
SelC800. The slot number is passed in the [A] register on a JSR to the entry
point at $1922. (See the subroutine listing in table 9.) A value of $00
deselects all slots. Note that SelC800 saves the slot number in $DFC0;
this allows the interrupt manager to restore the proper card allocation
should an interrupt occur. The interrupt manager routinely deselects all
slots on entry and reselects the proper slot on the way out.

The documentation states that SelC800 may be called from any environment
including interpreters (except an NMI handler). This turns
out not to be entirely true. The subroutine builds an instruction on-the-fly
by storing the slot number ORA #C0 as high byte of the operand in
bit $C0FF. The bit instruction then physically enables $C800 space for
that slot. But this area of SOS is write-protected while running in the user
environment, so the STA instruction doesn't work and the subroutine
fails without notifying you. If you want to call SelC800 from the user
environment you must enable write by clearing bit 3 of the environment
register ($FFDF).

There must be another soft switch somewhere. When you enter the
Monitor (with control-open-apple-reset), it comes up in forty-column
mode. You can change to eighty-column mode with escape-8, and back
again with escape-4. From eighty-column mode you might suppose you
could also change back to forty columns by fiddling with the soft
switches, perhaps by reading $C052 and maybe $C054. Things change,
and you can tell it's really trying hard. But no combination quite makes
it. We don't know why.

System Internal Resources: SIRs. When an interrupt occurs, the
interrupt manager must know which interrupt handler goes with which
interrupting device and where the handler address is located in memory.
The SIR allocation scheme provides a look-up table. It also establishes
"ownership" of a resource in order to prevent squabbles. Resources
should therefore be allocated whether there will be interrupts or not.
Somewhere in your code, place the following data table:

Allocation is performed by JSR AllocSIR ($1913) and deallocation
by JSR DealcSIR ($1916). The 6502 registers must contain: X =
SIRADDR; Y = SIRADDR+ 1; A = total number of bytes in
SIRTABLE. This will be $05, or some multiple of $05 in the event that
several resources are allocated at the same time. AllocSIR returns with
carry clear if the resource is successfully allocated.

Table 10 lists the numbers assigned to various resources. Examination
of AllocSIR suggests that the range is $00..17. There are a lot of
question marks. One wonders about the digital/analog audio converter,
the paddle ports, and other mysteries, such as whether the interrupt line
of the MM58167A clock chip is wired up.

Boot Sequence: On power-up, or after control-reset, the boot process
begins in ROM#1 (ROM#2 doesn't yet exist). Low-level diagnostics
are performed. Then block 0 is read from the disk in the built-in drive.
This is the SOS boot code and is present on every disk that has been formatted
by the System Utilities program. It must be present for a successful boot.
It consists of one block of "absolute" code and is loaded into
the computer at $A000, where it begins to run.

The boot code begins by locating and switching in the highest bank
of RAM. Then it goes back to the disk and loads in five more blocks
(blocks 1..5). These are placed in $A200..ABFF. Block 1 currently contains
all zeros; blocks 2..5 are the disk directory. The boot code then
scans the directory and locates SOS.Kernel, which it loads into memory
at $1E00..73FF.

When SOS.Kernel begins running, it promptly relocates bytes
$3000..73FF into the area $BC00..FFFF. This is the functional
SOS.Kernel. The loader portion is eventually overwritten and discarded.
First, however, it locates and loads SOS.Interp and loads the drivers
from SOS.Driver. It then initializes SOS.Kernel and each of the drivers.
Finally, control is transferred to the first instruction in the interpreter and
you are in business.

Data Disks: Your Own Boot Code. If you want to end up in Apple
III native mode, the boot process had better find the SOS boot code in
block 0 on the disk in the built-in drive. Any disks you ever expect to use
as SOS boot disks must have that code. On the other hand, you may
wish to create data disks that have the SOS directory structure but cannot
be booted. Or you may want the disk to boot, but to end up with
some entirely different operating system in the machine, such as an emulator,
for example. For either of these alternatives you will want to put
your own code in block 0 on your disk.

You start with a single block: the 512 bytes contained in block 0. It
will be loaded and begin to run at $A000. You may then use the ROM
subroutines to load in more blocks, so you can actually requisition as
much space as you require. At the time your code begins to run, you will
be in the Monitor or something very similar. The environment register
reads $77, zero page is $03, and bank 0 is switched in. You have available
all the hardware, including the VIA registers, extended addressing
(with proper zero page), and all the internal resources. You do not, of
course, have any of the SOS subroutines and facilities.

Your code should be assembled as "absolute" code by the Pascal
6502 assembler and should be ".org'd" on $A000. For data disks it
should end in an infinite loop. Word Juggler, for example, creates data
disks that, when you try to boot them, print "Can't boot Word Juggler
data disk" in the middle of the screen and politely hang the computer.

Formatting Data Disks. After you've assembled your own boot
code, it is a relatively simple matter to format data disks from application
programs. It can be done entirely from Pascal and almost entirely
from Basic. From Basic you will also need an invokable module that will
write to a floppy disk by block number, just as Unitwrite will do in Pascal.
The assembly source text for such a module is appended at the end of
this article.

The floppy format driver (FMTDX) is activated by issuing a DControl
call, code number 254 ($FE), to the appropriate driver (.FMTD1 for
the built-in drive). In Pascal this is done with Unitstatus procedure. In
Basic you use the Request.Inv invokable module that comes on the Apple
III Basic boot disk. If you're working in assembly you just issue SOS call
$83. When the DControl call is issued, the format process begins
immediately. All error checking and confirmation requests must be done by
your program before you issue the call.

The DControl call must specify a control list buffer. FMTDX.Driver
expects a one-page (256-byte) buffer that will be reproduced on each page
of the new disk. Normally this buffer should contain all zeros. The formatter
places address code on each track and sector and fills the data
fields with zeros, or whatever you put in your buffer. Then it quits.

You now have a formatted disk. It is not yet a SOS disk. It contains
neither a directory nor the block 0 boot code. You must store those yourself
from program buffers using Unitwrite (if you are working in Pascal).
If you ever want to use the disk as a SOS boot disk, just copy block
0 from some other boot disk. Otherwise transfer your own code. Remember
to chop off the header block, which the assembler will have
placed in front of your code. Start the transfer at block 1 of the codefile.

Next you must install a directory. The minimum requirements for a
usable SOS directory are listed in table 11. You must store the indicated
byte values on the disk. Just put them in the proper place in the 512-byte
buffer and write the whole block onto the disk.

Word Juggler manages to write all this information onto the disk by
a short segment of elegant and compact code. The utilities program uses
the brute-force approach. It simply includes fourteen pages of a standard
SOS structure (mostly zeros) and transfers the whole thing to disk
in one piece. No wonder the utilities program is 123 blocks long.

Unitread and Unitwrite for Basic: an Invokable Module. The
Device.IO.Inv invokable module contains two procedures. In Basic they
are external procedures and require the perform statement (see page 162,
Apple Business Basic Manual).

These procedures read from or write to a specified block number
(block%) on the disk in a specified device number (devnum%). They
transfer (length%) bytes to or from the buffer in memory. The buffer
must contain enough bytes or Unitread will spill data over onto surrounding
memory with disastrous results. Normally the buffer should be
dimensioned as an integer array; for example, DIM buf%(512). This
buffer will contain more than enough room for two blocks (1,024 bytes).

Unitwrite is a dangerous procedure. There is absolutely no protection
from errors. It is easy to write all over a disk directory, destroying it
and rendering the entire disk unusable.

The device numbers of the floppy disks are .D1 = 1, .D2 = 2, and
so on.

After typing in the text, save it to any pathname, perhaps Devio.Text.
Then assemble it to the corresponding codefile, Devio.Code. Finally,
change the name to Device.IO.Inv. If the assembler is not allowed to append
the suffix .code, the file-type designation will get all screwed up and
it won't invoke.